directsoundsink: Fix corner case causing large CPU usage

We were unnecessarily looping/goto-ing repeatedly when we had exactly
the amount of data as the free space, and also when the free space was
too small. This, as it turns out, is a very common scenario with
Directsound on Windows.
Fixes https://bugzilla.gnome.org/show_bug.cgi?id=773681
We have to do polling here because the event notification API that
Directsound exposes cannot be used with live playback since all events
must be registered in advance with the capture buffer, you cannot
add/remove them once playback has begun. Directsoundsrc had the same
problem.
See also: https://bugzilla.gnome.org/show_bug.cgi?id=781249